There are a few possible reasons.
If you’re using an Android device, you might get an error message because you have antivirus software such as Android AVG or parental control app Qustodio installed on the device which is preventing a secure connection within the Smart-ID app. Try disabling the software and registering again to see if it solves the problem.
You might also get the error message “connection is not secure” if you are using mobile internet and have run out of allocated data. Check if you can still use the internet and if not, either connect to a Wi-fi network or contact your mobile provider to top up your data.